Quartz to glass seal

ABSTRACT

A seal is provided for sealing a quartz envelope to a glass stem in an electron tube. The seal includes an intermediate annular metallic stress relieving ring disposed between the quartz envelope and the glass stem. The annular metallic stress relieving ring has: a radial flange portion, plated with a noble metal and sealed to a silver plated surface of the quartz envelope by a silver chloride cement; a supporting thin narrow relieving portion connected and extending substantially perpendicular to the radial flange portion; and a stem sealing portion which is sealed to the glass stem by conventional sealing techniques and which is interconnected with the annular stress relieving portion.

[ 1 Dec. 2, 1975 United States Patent [1 1 Faulkner l l QUARTZ TO GLASS SEAL Primary ExaminerBilly S. Taylor [75] Inventor:

Richard Dale Faulkner, Lancaster Attorney, Agennor Firm-G. H. Bruestle; D. 5. Cohen; R. .I. Boivm [57] ABSTRACT A seal is provided for sealing a quartz envelope to a [73] Assignee: RCA Corporation, New York, NY.

Filed: July 12, 1973 Appl. No.: 378,516 glass stem in an electron tube. The seal includes an intermediate annular metallic stress relieving ring dis- 1 U S Cl 220/2 3 313/220 313/721 posed between the quartz envelope and the glass stem.

The annular metallic stress relieving ring has: a radial 403/28; 403/29 HOlj 17/18; HOlj 61/36 flange portion, plated with a noble metal and sealed to [5 l] Int.

a silver plated surface of the quartz envelope by a sil- [58] Field of Search................... 220/2.l R, 2.3, 2.2;

ver chloride cement; a supporting thin narrow relieving portion connected and extending substantially perpendicular to the radial flange portion; and a stem sealing portion which is sealed to the glass stem by conventional sealing techniques and which is interconnected with the annular stress relieving portion.

220/23 UX 220/2.3 X

3 Claims, 4 Drawing Figures \7///////////// m 6 H) r A F 8 6 2 2 l l 3 3 U.S. Patent Dec. 2, 1975 QUARTZ TO GLASS SEAL BACKGROUND OF THE INVENTION The present invention relates to quartz to glass seals and more particularly to strong hermetic seals-between quartz envelopes and glass stems in electron discharge devices.

Conventional glass to glass or glass to metal seals for electron discharge devices generally utilize a direct heating and fusing process which is well known and does not present unusual difficulties. However, in special purpose tubes such as photomultipliers, it is often desirable to use quartz envelopes which are transmissive to particular invisible radiation wavelengths or which provide reduced radioactive background noise. The permanent sealing of a quartz envelope to a glass stem is exceedingly difficult because of stresses exerted on the seal by thermally caused dimensional changes in apposing materials having differing temperature expansion characteristics and normal external forces which are exerted on the stem region of the device.

One solution for this problem has been to use stepped layers of glass having specifically designed varying thermal properties to provide a graded seal between a quartz faceplate portion and the stem portion of the envelope. This type of graded sealed envelope, however, is expensive and is inadequate for certain special purpose devices which require that the envelope be primarily of quartz. Another solution has utilized a deformable metallic sleeve which is sealed by adhesive layers. As disclosed in US. Pat. No. 3,243,072, issued to C. L. Day on Mar. 29, 1966, this construction has been considered to be inadequate for many applications. Such seals have generally been utilized for specially constructed devices wherein the deformable metal sleeve is non-supporting and protected from external forces. Also, many sealing methods, in general, produce gases which tend to contaminate the active elements of the device thereby limiting the performance of assembled devices.

SUMMARY OF THE INVENTION The novel quartz-to-glass seal for sealing a quartz envelope to a glass stem in an electron tube, wherein a thin annular metallic stress relieving ring is intermediately disposed between the quartz envelope and the glass stem. The ring includes a radial flange portion, which is sealed to the open end of the quartz envelope by a suitable cement. A supporting thin narrow annular stress relieving portion on the ring is connected and extends substantially perpendicular to the radial flange portion. The ring also includes a stem sealing portion which is sealed to the glass stem by conventional sealing methods.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cutaway view of a photomultiplier tube.

FIG. 2 is a cross-sectional view of the tube of FIG. 1 taken along line 2-2.

FIG. 3 is an exaggerated cutaway view of the quartzto-glass seal of the tube of FIG. 1 in accordance with the invention.

FIG. 4 is a cross-sectional view of the quartz-to-glass seal in accordance with the invention taken along line 44 of the tube shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2,-ther c is shown a circular cage photomultiplier tube 10 in which light is focused, from source S. through a side of quartz envelope 1-2 and electron permeable grid 13 onto a photocathode surface 14 for generating electrons. In the operation of the tube 10, electrons emitted from photocathode surface 14 are repeatedly accelerated and multiplied by secondary electron ernissive surfaces 16, in a conventional manner, and ultimately collected as a signal current at anode 18. The electrodes of the tube 10 are connected to external potentials by means of leadin pins 20 which are hermetically sealed in a glass stem 22. In the assembly of the tube 10, prior to activation of the photocathode surface 14, the glass stem 22 must be hermetically sealed to the quartz envelope 12. The invention, disclosed herein, is directed to an improved seal between stem 22 and quartz envelope 12.

As shown in FIG. 3, the glass stem 22 is disc-shaped and includes an exhaust tubulation 24, approximately centrally located, and an outer peripheral sealing surface 26. The glass stem 22 is sealed indirectly tothe quartz envelope 12 through an intermediate stress relieving ring 28. Asshown in FIGS. 3 and 4, the stress relieving ring 28 comprises three portions: an annular stem sealing portion' 30 which is sealed to stem sealing surface 26, a radial flange portion 32 which is sealed to the quartz envelope l2 and an intermediateannular metallic stress relieving portion 34 which comprises an intermediate zone of yieldablesupporting metal.

The peripheral stem sealingsurface 26 is sealed to the annular stem sealing portion 30 by conventional sealing techniques such as, for example, RF sealing or flame sealing methods. Alternatively,the glass stem 22 may be molded into sealing relationship with the annular stem sealing portion 30. In general, the shape of the stem sealing portion 30-may vary with the type of seal utilized and is not considered critical; The stem 22 consists of a suitable glass, preferably a borosilicate glass, having substantially the same coefficient of thermal expansion as the stress relieving ring 30. In general, the material of the stem22 and the stress relievingring 30 are considered to possess substantially'the same coefficient of thermal expansionwhenever the differences in thermally caused dimensional changes in-the two opposing materials is insufflcient to cause cracking of the vacuum seal. The seal provided between the annular stem sealing portion 30 of the stress relieving ring 28 and the peripheral stern sealing surface 26 must be vacuum tight and of sufficient strength to withstand external forces normally exerted in the stem region of the tube.

The stress relieving ring 28, preferably consists of a single thickness of thinKovar (an alloy of cobalt, nickel and iron) material, but may consist of other materials of varying thicknesses having equivalent ductile properties such as: molybdenum, tungsten, or other alloys of nickel, cobalt and iron.

In general, the construction of stress relieving ring 28 is subject to several constraints. The thickness of material selected is critical, particularly with regard to intermediate metallic stress relieving portion 34. The thickness of the material must not be such as to make the stress relieving portion 34 exceedingly rigid. The material must be sufficiently yieldable and flexible to withstand stresses arising from thermally caused dimension changes of the quartz envelope l2 and the stress relieving ring 28 relative to each other. Variables which must be considered when selecting the appropriate thickness of the metallic material for the stress relieving portion 34 include, for example, the diameter and thickness of the quartz envelope. ln contrast, however, the stress relieving portion 34 must also be of sufficient strength and rigidity to withstand atmospheric pressure as well as other normal external forces which may be exerted on the stem region of the tube. In this regard, the requisite degree of strength and rigidity is obtained by providing an intermediate stress relieving portion 34 which is relatively narrow in width "W along the longitudinal length of the tube (measured in the direction of axis AA depicted in FIG. 1) as depicted in FIG. 3, exclusive of the stem sealing portion 30. Additional strength is obtained along longitudinal length of the tube by making the stress relieving portion 34 a supporting member which effectively extends the walls of the quartz envelope 12 to the peripheral stem sealing portion 30.

In accordance with the constraints stated above, the chart given below indicates, approximately, the various preferable thickness (T) and longitudinal widths (W) for a Kovar stress relieving portion 34 for various diameter quartz envelopes (D) and various conventional thicknesses of quartz envelopes (Q).

STRESS RELlEVlNG PORTION OF THE STRESS RELlEVlNG QUARTZ ENVELOPE Other, suitable values for these given variables, in the case of the other materials referred to earlier, may be extrapilated, by persons skilled in the art, in a manner which compensates for the varying degrees of yieldability, strength, and size of each material.

As shown in FIG. 1, a radial flange portion 30 which extends in a radial direction relative to the axis A-A depicted in FIG. 1, and which is substantially perpendicular to the intermediate stress relieving portion 34, is sealed to the annular surface 36 of the open end of the quartz envelope 12.

Prior to the assembly of tube 10, a sealing surface 38 on the radial flange portion 30 is cleaned and plated with a noble metal material such as, for example, silver, to prevent decomposition, by the base metals in the sealing surface 38, ofa silver chloride cement 40 which is utilized to accomplish the seal with surface 36 of envelope 12. Also, in order to improve the wetting characteristics of the quartz envelope 12 with the silver chloride cement, the annular surface 36 of the open end of the envelope [2 is plated with a noble metal, such as, for example, silver. Silver chloride is then applied to the surface 36 of envelope 12 by dipping that surface into molten silver chloride. The seal between the quartz envelope 12 and radial flange portion 32 is 4 accomplished by placing the two parts together and heating to the melting point of silver chloride (456C. then cooled.

The seal described above has been found particularly useful for tubes of 0.5 inch to 2.0 inches in diameter. Also, it has been found that a strong simple inexpensive hermetic seal is accomplished whereby differences in the thermally caused dimensional changes between the quartz envelope and its associated annular stress relieving ring, over a temperature range of 400C. to 0C. are yieldable resisted without cracking of the seal. A seal is accomplished which will not collapse under atmospheric pressure, temperature variations between 400C. and 0C., and/or normal external forces which are exerted in the stem region of the tube. Also, assembly of devices with the novel seal does not produce gases which tend to contaminate active elements internal of the assembled devices, such as for example, photocathodes.

while only a single embodiment has been illustrated for the seal between the quartz envelope 12'and the stress relieving ring 28, numerous variations of that seal may be accomplished by omitting changing or adding other materials which accomplish similar overall functions. For example, a cement other than silver chloride might be utilized without deviating from the inventive concept. Thus, the cement, while preferably silver chloride, may comprise other suitable adhesives having comparable characteristics such as, for example, the yieldability and pliability, which are required to provide a flexible and yielding vacuum seal between the opposing quartz and metallic material.

What I claim is:

l. A quartz-to-glass seal for sealing an annular open end portion of tubular quartz envelope to a glass stem in an electron tube wherein the annular open end of the envelope has an approximate outside diameter within the range of 0.5 to 2.0 inches; and wherein the seal includes an intermediate metallic stress relieving ring, between the annular open end portion of the envelope and the glass stem, of a material selected from the group consisting of: molybdenum, tungsten, and alloys of cobalt, iron, and nickle, and having:

a. a flange portion with a radial sealing surface plated with a noble metal which is sealed to the annular open end of the quartz envelope by a yieldable cement;

b. a supporting thin narrow tubular stress relieving portion having an approximate longitudinal width within the range of from 0.183 to 0.25 inches, an approximate thickness of between 5 and 15 mils; said relieving portion extending substantially perpendicular from said radial flange portion as a tubular extension of said envelope, and

c. a stem sealing portion extending from said tubular stress relieving portion and sealed to said glass stem.

2. A quartz-to-glass seal in accordance with claim 1 wherein said sealing surface on said radial flange portion comprises a surface plated with silver.

3. A quartz-to-glass seal in accordance with claim 2,

said yieldable cement comprising silver chloride. 

1. A QUARTZ-TO-GLASS SEAL FOR SEALING AN ANNULAR OPEN END PORTION OF TUBULAR QUARTZ ENVELOPE TO A GLASS STEM IN AN ELECTRON TUBE WHEREIN THE ANNULAR OPEN END OF THE ENVELOPE HAS AN APPROXIMATE OUTSIDE DIAMETER WITHIN THE RANGE OF 0.5 TO 2.0 INCHES; AND WHEREIN THE SEAL INCLUDES AN INTERMEDIATE METALLIC STRESS RELIEVING RING, BETWEEN THE ANNULAR OPEN END PORTION OF THE ENVELOPE AND THE GLASS STEM, OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF: MOLYBDENUM, TUNGSTEN, AND ALLOYS OF COBALT, IRON, AND NICKEL, AND HAVING: A. A FLANGE PORTION WITH A RADICAL SEALING SURFACE PLATED WITH A NOBLE METAL WHICH IS SEALED TO THE ANNULAR OPEN END OF THE QUARTZ ENVELOPE BY A YIELDABLE CEMENT; B. A SUPPORTING THIN NARROW TUBULAR STRESS RELIEVING PORTION HAVING AN APPROXIMATE LONGITUDINAL WIDTH WITHIN THE RANGE OF FROM 0.183 TO 0.25 INCHES, AN APPROXIMATE THICKNESS OF BETWEEN 5 AND 15 MILS; SAID RELIEVING PORTION EXTENDING SUBSTANTIALLY PERPENDICULAR FROM SAID RADICAL FLANGE PORTION AS A TUBULAR EXTENSION OF SAID ENVELOPE, AND C. A STEM SEALING PORTION EXTENDING FROM SAID TUBULAR STRESS RELIEVING PORTION AND SEALED TO SAID GLASS STEM.
 2. A quartz-to-glass seal in accordance with claim 1 wherein said sealing surface on said radial flange portion comprises a surface plated with silver.
 3. A quartz-to-glass seal in accordance with claim 2, said yieldable cement comprising silver chloride. 